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Demystifying Free-Space Path Loss
FSPL is the floor of every link budget and the first sanity check on any RF safety analysis. Here's the equation, the assumptions baked into it, and the cases where it lies.
The equation#
FSPL (dB) = 20·log10(d) + 20·log10(f) + 32.44Where d is in km and f is in MHz. The 32.44 constant absorbs the 4π and the unit conversions.
What it assumes#
Free space. No ground reflection, no atmospheric absorption, no diffraction, no multipath.
Why RF safety still uses it#
Because it's conservative for safety analysis: the real path loss in any cluttered environment is higher, so the safety distance computed under FSPL assumptions is always a worst-case bound. The FSPL calculator on the tools page returns both the loss and the implied safe distance.
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